An exhaust gas turbocharger is a type of forced induction system in which engine exhaust gases drive a turbine wheel. The turbine wheel is connected via a shaft to a compressor impeller. Ambient air is compressed by the compressor impeller and is fed into the intake manifold of the engine, allowing the engine to combust more fuel, and thus to produce more power for a given displacement. Considering the volumetric gas intake requirements of an engine operating at peak performance and the comparatively small size of a turbocharger, it can be appreciated that a turbocharger may be expected to rotate at speeds of up to 300,000 rpm and higher. In addition, the engine exhaust gas that drives the turbine wheel may have a temperature as high as 1,300 F. Thus, turbochargers generally operate at extremely high rotational speeds, and under conditions of high temperature and varying load.
The shaft is supported by a bearing system that includes a single or two spaced-apart journal bearings, which function to stabilize the shaft and dampen oscillations. The bearing system is cooled using a lubrication system in which oil is channeled through the bearing system for removal of heat. The oil flow behavior between the shaft and the inner diameter of the journal bearing can influence the rotordynamics of shaft motion and transmitted vibrations.